Submicrometer structures formed at liquid–liquid interfaces fundamentally alter interfacial hydrodynamics by modifying interfacial tension, generating Marangoni stresses, and inducing surface viscoelasticity. In many multiphase systems, interfacial materials are formed in situ through emulsification, surfactant assembly, or gelation processes, leading to the emergence of structured, viscoelastic interfacial layers. This presentation focuses on how such in-situ interfacial material formation alters droplet pinch-off dynamics and filament thinning at oil–water interfaces. I will discuss the break-up of liquid filaments in systems where interfacial layers are formed dynamically during flow. While Newtonian threads exhibit self-similar power-law thinning, the rapid formation of viscoelastic interfacial layers shifts the dynamics toward an exponential, elasto-capillary regime. Particular emphasis will be placed on the role of internal phase gelation and on the contrasting interfacial response of soft particles versus rigid particles. Soft, deformable structures can reorganize and couple with interfacial stresses, leading to delayed pinch-off and extended filament lifetimes, whereas hard particles produce distinctly different thinning behavior due to limited interfacial compliance. These results demonstrate how interfacial rheology governs the stability and breakup of liquid–liquid–solid systems. In the second part of the presentation, I will discuss the design of complex fluid systems for subsurface energy extraction, with emphasis on geothermal applications at the Utah FORGE site. By tailoring interfacial properties and multiphase flow behavior, we aim to engineer fluid formulations that improve transport, stability, and energy recovery in high-temperature geothermal environments.